Dipartimento di Astronomia, Universit`a di Bologna, Via Ranzani 1, I-40127 Bologna, Italy e-mail: silvia.pellegrini@unibo.it

Mem. S.A.It. Vol. 79, 270

SAIt 2008 c Memorie

The high energy emission of nearby supermassive black holes

S. Pellegrini

Dipartimento di Astronomia, Universit`a di Bologna, Via Ranzani 1, I-40127 Bologna, Italy e-mail: silvia.pellegrini@unibo.it

Abstract. The formation and evolution of the central massive black hole and of the host galaxy are intimately related; the understanding of how accretion (and its related feedback) work, though, and of their evolution over cosmic time, is still uncomplete. After the bright AGN phase accretion may switch to a radiatively inefficient mode, possibly associated with outflows, and/or accretion may be intermittent. I shortly present currently debated scenarios for the accretion modalities in the local Universe, and the role of Simbol-X to establish their importance.

Key words. Galaxies: elliptical and lenticular, cD – Galaxies: evolution – Galaxies: ISM – Galaxies: nuclei – X-rays: galaxies

1. Introduction

Supermassive black holes (MBH) are present at the center of the spheroids of the local uni- verse, with MBH∼ 10

− 10

M and im- portant correlations link MBHs and the host galaxies (Tremaine et al. 2002). Therefore the MBH birth, growth and activity cycle must be linked to the formation and evolution of the spheroid, however our understanding of how MBHs and galaxies coexist is still uncomplete.

For example, feedback from the MBH should regulate starformation at early epochs; in later epochs, feedback is required to solve the ”cool- ing flow” problem (Ciotti & Ostriker 2001;

Omma et al. 2004). Observationally the nuclei of most spheroids of the local Universe are ei- ther radiatively quiescent or show low levels of activity (Ho 2005). Chandra observations al- lowed to measure X-ray emission (a key symp-

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tom of accretion) from a significant number of nuclei [Fig. 1, Pellegrini (2005)].

Thanks to the angular resolution of Chandra, the mass accretion rate at the ac- cretion radius (∼few 10

Schwarzschild radii) within the Bondi theory could be determined;

numerical simulations of the hot ISM evolu- tion also show accreting flows [at a rate ˙ M ∼ 0.1 − 10

M /yr at the present epoch; e.g., Pellegrini et al. (2007)].

2. The Main Questions

1) How is the MBH activity switched off while the accretion rate decreases?

After the bright AGN phase, if the mass ac- cretion rate goes below ˙ M ∼ 0.01 ˙ M

, the standard disk may transit to an optically thin radiatively inefficient accretion flow [RIAF;

Quataert & Narayan (1999)]. This could con-

cern a few of the nuclei in Fig. 1, if the Bondi

Pellegrini: X-rays from Nearby SMBHs 271

Fig. 1. L

and ˙ M

measured from Chandra data [from the sample of Pellegrini (2005); red points from Soria et al. (2006)]. The dotted line is the predicted emission for a standard optically thick accretion disk, the dashed one the predicted emis- sion level for RIAFs without winds (see Sect. 2).

rate is close to the true accretion rate. Also intermittent AGN activity is suggested by hy- drodynamical simulations with nuclear feed- back (Ciotti & Ostriker 2001; Omma et al.

2004). Finally, scaled-down AGN emission, possibly highly obscured, has been studied for some cases [e.g., Pellegrini et al. (2007);

Panessa et al. (2007)].

2) The best studied MBH resides at the Galactic Center. Can its radiatively in- efficient model (Yuan, Quataert, & Narayan 2004) be considered a baseline model for local spheroids? Actually, early type galax- ies lack complicacies as central massive stars and/or supernovae linked to central starforma- tion. Therefore they are ideal laboratories for studying low accretion rate physics.

In the RIAF case, the hot thermal elec- trons produce bremsstrahlung (with kT∼few to 100 keV) and Inverse Compton of synchrotron photons (with photon index Γ ∼ 2) in the X-rays. Sensitivity at the hard X-rays up to 70 keV, such as that expected for Simbol-X, would allow us to determine reliably the spec- tral shape in this band for the first time. We

could then contrast the normal AGN scenario vs. the RIAF scenario. For Seyfert-like spec- tra we expect an X-ray slope of Γ ∼ 1.8, plus possibly a reflection hump, strong vari- ability on short timescale, Fe K line emis- sion at 6.4 keV (see De Cia et al., this meet- ing). However, from Fig. 1, ˙ M must be <<

M ˙

(due to feedback?) or emission must be highly obscured, as discussed in Pellegrini 2005; Pellegrini et al. 2007. For RIAFs, taking M ∼ ˙ ˙ M

of Fig. 1 at their outer boundary, and MBH from dynamical studies, one expects the hard X-rays dominated by Comptonized emission (if no wind), and by bremsstrahlung emission, with an inner ˙ M << ˙ M

, if a wind is important (Quataert & Narayan 1999).

Also, no variability is expected on scales < 1 yr (Di Matteo et al. 2000).

The feasibility has been checked for the SDD and CZT detectors with the Simbol-X data simula- tor at http://www.asdc.asi.it/simbol- x/index.php?page=/simulator/simbol-

x/index.php. The Simbol-X HPD of 10 − 15

(see documents at www.asdc.asi.it) implies that only the brightest nuclei of the Pellegrini (2005) sample can be studied, due to con- tamination from the collective emission of unresolved X-ray binaries for L

< 10

erg s

. Then, for a few nuclei with a 2–10 keV flux of > 10

erg cm

s

good quality spectra (so to discriminate between the pos- sibilities above) can be obtained in exposure times of ∼ 100 ksec.

References

Ciotti, L. & Ostriker, J.P. 2001, ApJ, 551, 131 Di Matteo, T., et al. 2000, MNRAS, 311, 507 Ho, L.C. 2005, Ap&SS 300, 219

Omma, H., et al. 2004, MNRAS, 348, 1105 Panessa, F., et al. 2007, A&A 467, 519 Pellegrini, S. 2005, ApJ, 624, 155

Pellegrini, S., et al. 2007, in press on ApJ (astro-ph/0701642)

Quataert, E., Narayan, R. 1999, ApJ, 520, 298 Soria, R., et al. 2006, ApJ640, 126

Tremaine, S., et al. 2002, ApJ, 574, 554 Yuan, F., Quataert, E., & Narayan, R. 2004,

ApJ, 606, 894